The recovery of ethylene from crude light hydrocarbon gas mixtures is an economically important but highly energy intensive process. Cryogenic separation methods are commonly used which require large amounts of refrigeration at low temperatures, and the continuing development of methods to reduce net power to provide this refrigeration is important in the petrochemical industry.
Ethylene is recovered from light gas mixtures such as cracked gas from hydrocarbon crackers which contain various concentrations of hydrogen, methane, ethane, ethylene, propane, propylene, and minor amounts of higher hydrocarbons, nitrogen, and other trace components. Refrigeration for condensing and fractionating such mixtures is commonly provided at successively lower temperature levels by ambient cooling water, closed cycle propylene and ethylene systems, and work expansion or Joule-Thomson expansion of pressurized light gases produced in the separation process. Numerous designs have been developed over the years using these types of refrigeration as characterized in representative U.S. Pat. 3,675,435, 4,002,042, 4,163,652, 4,629,484, 4,900,347, and 5,035,732.
The use of closed cycle mixed refrigerant systems can be integrated with one or more of the above-mentioned refrigeration methods to improve the overall energy efficiency of ethylene recovery. Mixed refrigerants for such systems typically comprise methane, ethane, ethylene, propane, propylene, and optionally other light components. Mixed refrigerants exhibit the desirable property of condensing over a range of temperatures, which allows the design of heat exchange systems which are thermodynamically more efficient than single refrigerant systems.
U.S. Pat. No. 4,072,485 describes a mixed refrigerant cycle for providing low level refrigeration in a natural gas processing plant, or in the cryogenic section of an ethylene plant which utilizes one or more partial condensation stages to cool the feed gas. In this cycle, the mixed refrigerant is partially condensed with cooling water or air at near ambient temperature and then totally condensed at +50.degree. F. and subcooled with several levels of propane or propylene refrigeration. In ethylene plant service, the mixed refrigerant is then utilized to provide refrigeration over the temperature range of -40.degree. F. to -148.degree. F.; i.e., it is confined to the same temperature range as the ethylene refrigeration it replaces. A more specific example of this cycle for ethylene plant service is described in an article by Victor Kaiser, et al., "Mixed Refrigerant for Ethylene," in the Oct. 1976 issue of Hydrocarbon Processing, pages 129-131.
U.S. Pat. 4,720,293 describes a process for recovering ethylene from refinery off-gas which utilizes a mixed refrigerant cycle. In this process, the mixed refrigerant is utilized in a single heat exchanger over a relatively warm temperature range of +60.degree. F. to -85.degree. F. Refrigeration at lower temperature levels is supplied by vaporization of separated ethane at low partial pressure and high total pressure, and by work expansion of light gases which are typically rejected to fuel along with the ethane.
With the conventional process technology described above, the feed gas chilling and demethanizing must be carried out at pressures in the range of 450 to 650 psia in order to achieve high ethylene recovery (994% or more) because the propylene/ethylene cascade system can provide refrigeration no colder than -150.degree. F. for feed gas chilling and for demethanizer column condenser refrigeration. The amount of refrigeration for feed cooling below -150.degree. F. which can be produced from other process streams in an ethylene plant is limited by operating constraints such as the amount of high pressure hydrogen recovered and the fuel system pressure(s). These constraints limit the amount of expander refrigeration which can be produced, which in turn limits the ethylene recovery. Pressures between 450 and 650 psia are required in the feed gas chilling train and in the demethanizer column so that most of the ethylene can be condensed above -150.degree. F., and so that sufficient fuel gas expansion refrigeration at colder temperatures is available to condense most of the remaining ethylene and achieve low ethylene loss in the demethanizer column overhead vapor.
The integration of improved mixed refrigerant cycles with conventional intermediate and low temperature refrigeration holds promise for further reduction of energy consumption in ethylene recovery. In particular, it is desirable to improve the efficiency of refrigeration at the lowest temperature levels required for high ethylene recovery. The invention described in the following specification and defined in the appended claims provides an improved mixed refrigeration cycle which is particularly advantageous for ethylene recoveries of greater than 99%.